1) One who kills members of their team in online multiplayer matches, causing much frustration to their victims. 2) The assholes of the gaming universe. 3) An individual playing online videogames, who joins the opposing team and intentionally gets themselves killed in order to help their friends on the other team win.

Billbot70: What? Why did he just frag me!?!

FreyedChicken67: You didn't know? He prides himself in being a Team Killer...

When a n00b is really crap at a game and they keep getting owned they'll kill their own team mates as its the only way to score any kills. More often than not they'll eventually get kicked from the server and everyone will remember how much of a twat they are.